I was dubious so did a fact check, and it is indeed necessary! Not sure what the reasoning is, I wonder if it's some rule about public swimming which was originally created only for pools? Or maybe there are some wildlife concerns with certain lotions/etc getting in the water.
> Y a-t-il des douches ? > > Oui ! Chaque site de baignade propose des douches – obligatoires avant d’aller se baigner –, un accès à des toilettes, un poste de secours ainsi que des espaces pour s’asseoir et profiter du soleil.
https://www.paris.fr/pages/baignade-en-seine-toutes-les-ques...
Maybe they're just erring on the side of caution since they just cleaned the place up.
I won't be surprised if the shower drains back into the river.
My guess is while there are few people swimming in the river now, it's easy to imagine there will be tens of thousands in a couple years. Paris-Plages is a huge hit. The city probably figures it's better to ease people into the habit rather than try to force it on everyone afterward.